The X-Tended Mod is a large addition to the existing X3:Reunion universe and is the only major mod (as of the 26th May 2010) that has been hosted by the creators of X3:Reunion, Egosoft, on their own servers. (Only 2 mods have been hosted by Egosoft in total.)
Many of the ships and ideas from XTM were added to the next game by Egosoft - X3:Terran Conflict. A lot of the XTM team also worked with Egosoft in creating Terran Conflict.
This mod massively expands the universe and the game itself, adding over 90 new ships, almost doubling the size of the universe, plus adding many new missions, stations, and things to find.

1. Make sure you have a clean install of X3:Reunion. (Windows Vista and
Windows 7 owners should be aware that a version of the game installed to
the 'program files' or 'program files(x86)' folders will not allow mods
to be added to the game correctly. Install the game to a custom folder like
c:\games\. This is an issue with all games on Windows Vista and Windows
7 and not a fault specifically with this mod or with X3:Reunion.)